EP2498114A1

EP2498114A1 - Objective lens for endoscope, and endoscope using same

Info

Publication number: EP2498114A1
Application number: EP10839206A
Authority: EP
Inventors: Yuko Katahira
Original assignee: Olympus Medical Systems Corp
Current assignee: Olympus Medical Systems Corp
Priority date: 2009-12-24
Filing date: 2010-12-10
Publication date: 2012-09-12
Also published as: CN102667570B; CN102667570A; WO2011077972A1; JPWO2011077972A1; US8449127B2; EP2498114A4; US20120154932A1; JP5031930B2

Abstract

Provided are an endoscope objective lens that has a simple configuration and is suitable for a compact, high-resolution image acquisition device usable in laser treatment and the like, and an endoscope using the same. Provided is an endoscope objective lens 1 including, in sequence from an object side, a front group (FG), an aperture stop (S), and a back group (BG). The front group (FG) includes, in sequence from the object side, a negative first lens (L1) whose concave surface faces an image side, a positive second lens (L2) whose convex surface faces the object side and whose flat surface or concave surface is located on the image side, and a filter (FL). The back group (BG) includes, in sequence from the object side, a positive third lens (L3) whose convex surface faces the image side, and a combined lens (E45) formed of a plano-convex lens or a biconvex lens (L4) and a negative meniscus lens (L5) and satisfies the following Conditional Expression (1)

n 3 > - ν 3 / 12 + 5.5

where n3 is the refractive index of the third lens (L3), and ν3 is the Abbe number of the third lens (L3).

Description

{Technical Field}

The present invention relates to an endoscope objective lens that has a simple configuration and is suitable for a compact, high-resolution image acquisition device usable in laser treatment and the like, and to an endoscope using the same.

{Background Art}

In endoscopes used in the medical field etc., in order to improve the operability and reduce stress on patients, it has been necessary to reduce the diameter of endoscope insertion portions and to reduce the length of rigid portions at the tip. Therefore, objective lenses installed therein must be configured to have a small outside diameter and a small overall length. Compact endoscope objective lenses having a simple configuration are known (for example, see PTLs 1 and 2).
On the other hand, in the case of endoscopes usable in laser treatment and the like, a filter, such as a laser-light cut filter or a color-correcting filter, needs to be inserted into the objective lens. Endoscopes having such filters are known (for example, see PTLs 1 and 3 to 5). In PTLs 1, 3, and 4, a filter is disposed near the image plane, not at a position immediately behind the aperture stop where the angle of incidence on the filter is large. In Example 9 in PTL 5, a filter is disposed in immediate proximity to the image plane. In Example 11 in PTL 5, filters are disposed immediately in front of the aperture stop and between the aperture stop and a combined lens disposed therebehind.
Meanwhile, in order to improve the diagnostic capability using an endoscope, it is important to improve image quality by correcting various optical aberrations. Endoscope objective lenses achieve a wide angle of view by employing a retrofocus structure, in which a lens group having negative refractive power is disposed on the object side of the aperture stop, and a lens group having positive refractive power is disposed on the image side of the aperture stop. However, because this configuration is asymmetrical with respect to the aperture stop, correcting lateral chromatic aberration is especially difficult. Objective lenses in which such lateral chromatic aberration is effectively corrected are known (for example, see PTL 6).

{Citation List}

{Patent Literature}

{PTL 1} The Publication of Japanese Patent No. 4245985
{PTL 2} Japanese Unexamined Patent Application, Publication No. 2007-249189
{PTL 3} The Publication of Japanese Patent No. 4229754
{PTL 4} The Publication of Japanese Patent No. 3574484
{PTL 5} Japanese Unexamined Patent Application, Publication No. 2004-354888
{PTL 6} Japanese Unexamined Patent Application, Publication No. 2007-249189

{Summary of Invention}

{Technical Problem}

However, the objective lenses in PTLs 1 and 2, which have a simple configuration, suffer from a problem of unbalanced aberrations, that is, for example, even if chromatic aberration is effectively corrected, field curvature, spherical aberration, and the like increase.
Furthermore, more-compact objective lenses having higher performance are required these days, in accordance with the development of more-compact image acquisition devices having higher resolution. When such a compact image acquisition device is used in an endoscope usable in laser treatment and the like, similarly to the case where a normal-size image acquisition device is used, a space for disposing an optical filter, such as a color-correcting filter, an infrared-cut filter, or the like, needs to be ensured inside the objective lens.
At this time, there is a first problem in that, if a filter is disposed in immediate proximity to the image plane, as in PTLs 1, 3, 4, and Example 9 in PTL 5, the filter should be disposed at the extreme rear end of an objective-lens holder or at the extreme front end of an image-acquisition-device holder. That is to say, because the filter is directly exposed to the outside, there is a risk of damaging (cracking, chipping, etc.,) the filter surface during assembly. A second problem is correcting the above-described lateral chromatic aberration. However, the lateral chromatic aberrations (the difference between the F line and the C line) occurring in Examples 9 and 11 in PTL 5 are large, i.e., 1.2% and 1.0% of the focal length, where the half angle of view ω is 60°, respectively. As described, with the objective lenses in PTLs 1 to 5, the first and second problems cannot be solved simultaneously.
On the other hand, Example 1 in PTL 6 effectively corrects lateral chromatic aberration (the difference between the F line and the C line), i.e., 0.04% of the focal length, where the half angle of view ω is 50°, with a small and simple optical system. However, there is obviously no space for disposing an optical filter, resulting in a problem in that it cannot be used in an endoscope that requires an optical filter.
The present invention has been made in view of the above-described circumstances, and an object thereof is to provide an endoscope objective lens that has a simple configuration and is suitable for a compact, high-resolution image acquisition device usable in laser treatment and the like, and an endoscope using the same.

{Solution to Problem}

To achieve the above-described object, the present invention provides the following solutions.
A first aspect of the present invention is an endoscope objective lens including, in sequence from an object side, a front group, an aperture stop, and a back group. The front group includes, in sequence from the object side, a negative first lens whose concave surface faces an image side, a positive second lens whose convex surface faces the object side and whose flat surface or concave surface is located on the image side, and a filter. The back group includes, in sequence from the object side, a positive third lens whose convex surface faces the image side, and a combined lens formed of a plano-convex lens or a biconvex lens and a negative meniscus lens and satisfies the following Conditional Expression (1) $n 3 > - ν 3 / 12 + 5.5$
where n3 is the refractive index of the third lens, and ν3 is the Abbe number of the third lens.
In the first aspect of the present invention, in order to achieve a compact endoscope with a short overall optical length, the number of constituent optical element is minimized, and a simple configuration composed of three groups, i.e., the front group, the aperture stop, and the back group, is employed. Furthermore, in order to achieve high image quality suitable for a high-resolution image acquisition device, it is arranged that the front group includes, in sequence from the object side, a negative first lens whose concave surface faces the image side, and a positive second lens whose convex surface faces the object side and whose flat surface or concave surface is located on the image side, and the back group includes, in sequence from the object side, a positive third lens whose convex surface faces the image side, and a combined lens formed of a plano-convex lens or a biconvex lens and a negative meniscus lens.
In addition, in order to achieve an endoscope usable in laser treatment and the like, it is arranged that an optical filter, such as a laser-light cut filter or a color-correcting filter, is disposed in the front group. In endoscope objective lenses in the related art, the filter or the like is usually disposed near the image plane, in the back group disposed on the image side of the aperture stop, not at a position immediately behind the aperture stop where the angle of incidence of light on the filter is large. However, with this configuration, the filter is located at the extreme rear end of the objective-lens holder, or at the extreme front end of the image-acquisition-device holder. That is, because the filter is directly exposed to the outside, there is a risk of damaging (cracking, chipping, etc.,) the filter surface during assembly. The reason why the filter is disposed in the front group is to avoid this risk.
Conditional Expression (1) defines the refractive index and Abbe number of the positive third lens in the back group. To correct chromatic aberration, it is preferable that the positive lens in the back group be composed of a material having a large Abbe number. By disposing the positive lens having small dispersion immediately behind the aperture stop, the lateral chromatic aberration can be effectively corrected. If Conditional Expression (1) is not satisfied, it is difficult to correct chromatic aberration in the entire system.
A second aspect of the present invention is an endoscope objective lens including, in sequence from an object side, a front group, an aperture stop, and a back group. The front group includes, in sequence from the object side, a negative first lens whose concave surface faces an image side, a positive second lens whose convex surface faces the object side and whose flat surface or concave surface is located on the image side, and a filter disposed between the first lens and the second lens. The back group includes, in sequence from the object side, a positive third lens whose convex surface faces the image side, and a combined lens formed of a plano-convex lens or a biconvex lens and a negative meniscus lens.
In the second aspect of the present invention, in order to achieve an endoscope usable in laser treatment and the like, an optical filter, such as a laser-light cut filter or a color-correcting filter, is configured to be disposed between the first lens and the second lens. This is to avoid a risk of damaging (cracking, chipping, etc.,) the filter surface during assembly. Furthermore, by disposing the filter or the like on the object side of the aperture stop, the angle of incidence of light on the filter is reduced. With this configuration, color correction and blocking of light having wavelengths in the infrared region can be effectively performed, and thus, this configuration is the preferred embodiment.
In the above-described second aspect, it is preferable that the following Conditional Expression (1) be satisfied $n 3 > - ν 3 / 12 + 5.5$
where n3 is the refractive index of the third lens, and ν3 is the Abbe number of the third lens.
In the above-described first and second aspects, it is preferable that the following Conditional Expressions (2) and (3) be satisfied $|f 3 / r 3| > 1.3$
$2.0 > df / Ih > 1.5$
where f3 is the focal length of the third lens, r3 is the image-plane-side radius of curvature of the third lens, df is the sum of the thickness of an optical element and the inter-surface distance from the apex of the concave surface of the first lens to the aperture stop, and Ih is the maximum image height.
Conditional Expression (2) defines the proportion of the focal length of the positive third lens in the back group to the radius of curvature of the image-side surface. In order to reduce the overall length and minimize variations in the angle of incidence on the image plane in a compact endoscope objective lens, light needs to be bent by a small number of lenses disposed on the rear side of the aperture stop. From the standpoint of the lens manufacturing and the assembly precision, it is desirable that the radius of curvature of the lens be larger than a certain value. Therefore, the focal length and the radius of curvature of the image-side surface of the third lens need to be balanced. If the lower limit of Conditional Expression (2), namely, 1.3, is not reached, although the angle of incidence on the image plane can be corrected so as to be parallel to the optical axis, because the radius of curvature of the image-side surface of the third lens increases, correcting chromatic aberration becomes difficult in the entire system, and lens processing conditions become severe.
In addition, in order to dispose the filter in the front group, a sufficient space in view of the size of the image acquisition device needs to be ensured in the front group. Conditional Expression (3) defines the proportion of the sum of the thickness of an optical element and the inter-surface distance from the apex of the concave surface of the first lens to the aperture stop to the maximum image height. If the lower limit of Conditional Expression (3), namely, 1.5, is not reached, it is difficult to ensure a sufficient space for disposing the filter or the like. If the upper limit 2.0 of Conditional Expression (3) is exceeded, although a space for disposing the filter or the like can be ensured, a need to increase the distance between the position behind the aperture stop and the image plane arises. As a result, the angle of incidence on the image plane increases, which may cause a shading phenomenon.
A third aspect of the present invention is an endoscope including any one of the above-described endoscope objective lenses.

{Advantageous Effects of Invention}

With the first to third aspects of the present invention, it is possible to provide an endoscope objective lens that has a simple configuration and is suitable for a compact, high-resolution image acquisition device usable in laser treatment and the like, and an endoscope using the same.

{Brief Description of Drawings}

{FIG. 1} FIG. 1 is a lens cross-sectional diagram showing the configuration of an endoscope objective lens according to an embodiment of the present invention.
{FIG. 2} FIG. 2 is a lens cross-sectional diagram showing the configuration of an endoscope objective lens according to Example 1.
{FIG. 3} FIG. 3 includes aberration diagrams showing spherical aberration, astigmatism, lateral chromatic aberration, coma in the M direction, and coma in the S direction of the endoscope objective lens in FIG. 2.
{FIG. 4} FIG. 4 is a lens cross-sectional diagram showing the configuration of an endoscope objective lens according to Example 2.
{FIG. 5} FIG. 5 includes aberration diagrams showing spherical aberration, astigmatism, lateral chromatic aberration, coma in the M direction, and coma in the S direction of the endoscope objective lens in FIG. 4.
{FIG. 6} FIG. 6 is a lens cross-sectional diagram showing the configuration of an endoscope objective lens according to Example 3.
{FIG. 7} FIG. 7 includes aberration diagrams showing spherical aberration, astigmatism, lateral chromatic aberration, coma in the M direction, and coma in the S direction of the endoscope objective lens in FIG. 6.
{FIG. 8} FIG. 8 is a lens cross-sectional diagram showing the configuration of an endoscope objective lens according to Example 4.
{FIG. 9} FIG. 9 includes aberration diagrams showing spherical aberration, astigmatism, lateral chromatic aberration, coma in the M direction, and coma in the S direction of the endoscope objective lens in FIG. 8.
{FIG. 10} FIG. 10 is a lens cross-sectional diagram showing the configuration of an endoscope objective lens according to Example 5.
{FIG. 11} FIG. 11 includes aberration diagrams showing spherical aberration, astigmatism, lateral chromatic aberration, coma in the M direction, and coma in the S direction of the endoscope objective lens in FIG. 10.
{FIG. 12} FIG. 12 is a lens cross-sectional diagram showing the configuration of an endoscope objective lens according to Example 6.
{FIG. 13} FIG. 13 includes aberration diagrams showing spherical aberration, astigmatism, lateral chromatic aberration, coma in the M direction, and coma in the S direction of the endoscope objective lens in FIG. 12.
{FIG. 14} FIG. 14 is a lens cross-sectional diagram showing the configuration of an endoscope objective lens according to Example 7.
{FIG. 15} FIG. 15 includes aberration diagrams showing spherical aberration, astigmatism, lateral chromatic aberration, coma in the M direction, and coma in the S direction of the endoscope objective lens in FIG. 14.
{FIG. 16} FIG. 16 is a lens cross-sectional diagram showing the configuration of an endoscope objective lens according to Example 8.
{FIG. 17} FIG. 17 includes aberration diagrams showing spherical aberration, astigmatism, lateral chromatic aberration, coma in the M direction, and coma in the S direction of the endoscope objective lens in FIG. 16.
{FIG. 18} FIG. 18 is a lens cross-sectional diagram showing the configuration of an endoscope objective lens according to Example 9.
{FIG. 19} FIG. 19 includes aberration diagrams showing spherical aberration, astigmatism, lateral chromatic aberration, coma in the M direction, and coma in the S direction of the endoscope objective lens in FIG. 18.
{FIG. 20} FIG. 20 is a lens cross-sectional diagram showing the configuration of an endoscope objective lens according to Example 10.
{FIG. 21} FIG. 21 includes aberration diagrams showing spherical aberration, astigmatism, lateral chromatic aberration, coma in the M direction, and coma in the S direction of the endoscope objective lens in FIG. 20.
{FIG. 22} FIG. 22 is a lens cross-sectional diagram showing the configuration of an endoscope objective lens according to Example 11.
{FIG. 23} FIG. 23 includes aberration diagrams showing spherical aberration, astigmatism, lateral chromatic aberration, coma in the M direction, and coma in the S direction of the endoscope objective lens in FIG. 22.
{FIG. 24} FIG. 24 is a lens cross-sectional diagram showing the configuration of an endoscope objective lens according to Example 12.
{FIG. 25} FIG. 25 includes aberration diagrams showing spherical aberration, astigmatism, lateral chromatic aberration, coma in the M direction, and coma in the S direction of the endoscope objective lens in FIG. 24.
{FIG. 26} FIG. 26 is a lens cross-sectional diagram showing the configuration of an endoscope objective lens according to Example 13.

{Description of Embodiments}

Referring to FIG. 1, an endoscope objective lens 1 according to an embodiment of the present invention will be described below.
As shown in FIG. 1, the endoscope objective lens 1 according to this embodiment is formed of, in sequence from the object side, a front group FG, an aperture stop S, and a back group BG.
The front group FG includes, in sequence from the object side, a negative first lens L1 whose concave surface faces the image side, a positive second lens L2 whose convex surface faces the object side and whose flat surface is located on the image side, and a filter FL disposed between the first lens L1 and the second lens L2. Any filter, such as a laser-light cut filter, a color-correcting filter, a transmission filter, an absorption filter, a reflection filter, or a polarizing filter, may be appropriately used as the filter FL.
Sapphire is preferably used as the glass material of the first lens L1. Sapphire is advantageous in correcting lateral chromatic aberration because it has a high Abbe number (νd = 71.79) and is also advantageous in correcting coma because it has a high refractive index (nd = 1.7682). In addition, sapphire is preferable for the first lens L1, which is externally exposed, because it is tolerant of high-temperature, high-pressure steam sterilization, called autoclave sterilization, and of chemicals used in chemical cleaning, which is usually performed in the use of endoscopes. A glass material having high sterilization resistance and chemical resistance, such as zirconia, Yttria-stabilized zirconia, synthetic quartz, transparent YAG, or spinel, may be used instead of sapphire.
The back group BG includes, in sequence from the object side, a positive third lens L3 whose convex surface faces the image side, and a combined lens E45 formed of a plano-convex lens or a biconvex lens (in the example shown, a plano-convex lens) L4 and a negative meniscus lens L5. The back group BG satisfies the following Conditional Expression (1) $n 3 > - ν 3 / 12 + 5.5$
where n3 is the refractive index of the third lens L3, and ν3 is the Abbe number of the third lens L3.
The endoscope objective lens 1 also satisfies the following Conditional Expressions (2) and (3) $|f 3 / r 3| > 1.3$
$2.0 > df / Ih > 1.5$
where f3 is the focal length of the third lens L3, r3 is the image-plane-side radius of curvature of the third lens L3, df is the sum of the thicknesses of the optical element (the filter FL and the second lens L2) and the inter-surface distance from the apex of the concave surface of the first lens L1 to the aperture stop S, and Ih is the maximum image height.
The thus-configured endoscope objective lens 1 can reduce the risk of damaging the filter FL during assembly by disposing the optical filter FL, which is a laser-light cut filter or the like, between the lenses L1 and L2. Furthermore, by simultaneously satisfying Conditional Expressions (1) to (3), a compact configuration can be achieved while effectively correcting all optical aberrations, including lateral chromatic aberration, in a balanced manner. Therefore, the endoscope objective lens 1 can be suitably used in an endoscope having a compact, high-resolution image acquisition device usable in laser treatment and the like. Furthermore, by disposing the filter FL closer to the object side, the angle of incidence of light on the filter FL is reduced. Thus, color correction and blocking of light having wavelengths in the infrared region can be more effectively performed.
In the above-described embodiment, the back group BG may include a plurality of lenses having positive refractive power. For example, a sixth lens having a positive refractive index may be used as an optical component 3. This configuration is advantageous in correcting field curvature because the angle of incidence on the image plane can be corrected so as to be parallel to the optical axis by disposing a lens having positive refractive power near the image plane.
Furthermore, in the above-described embodiment, an optical-path changing device, such as a prism or the like, may be disposed somewhere in the optical path.
For example, a prism is disposed on the object side of the image acquisition device to change the optical path in a direction perpendicular to the optical axis. By doing so, even when a large image acquisition device is used, an image acquisition surface of the image acquisition device can be disposed parallel to the optical axis, and the diameter of the tip of the endoscope can be reduced. Note that the direction in which the optical path is changed is not limited to a direction perpendicular to the optical axis.
Furthermore, in the above-described embodiment, the filter FL may be disposed behind the second lens L2 in the front group FG. Also with this configuration, similarly to the above-described embodiment, damage to the filter FL can be prevented, and a reduction in size can be achieved while effectively correcting various aberrations.

{Examples}

Next, examples of the above-described embodiment will be described below with reference to FIGS. 2 to 26. In the lens cross-sectional diagrams, r represents the radius of curvature, d represents the inter-surface distance, and the number following r or d represents the surface number. In the respective aberration diagrams, (a) represents spherical aberration, (b) represents astigmatism, (c) represents lateral chromatic aberration, (d) represents coma in the M direction, and (d) represents coma in the S direction. Furthermore, the aberration diagrams show aberrations for the d line (587.56 nm), serving as the reference wavelength, and aberrations for the C line (656.27 nm), the F line (486.13 nm), and the g line (435.83 nm) are also shown in the diagrams showing spherical aberration, lateral chromatic aberration, and coma. Furthermore, for coma, coma in a ray direction (M direction) and coma in a concentric circle direction (S direction) are shown. The refractive indices listed in the lens data are the refractive indices for the d line.

Example 1

FIG. 2 shows the configuration of an endoscope objective lens according to Example 1, and the lens data thereof are shown below. FIG. 3 shows aberration diagrams of the objective lens according to this example.
The endoscope objective lens according to this example is formed of, in sequence from the object side, a front group, an aperture stop, and a back group. The front group is formed of, in sequence from the object side, a negative first lens whose concave surface faces the image side, a filter, and a positive second lens whose convex surface faces the object side and whose flat surface is located on the image side. The back group is formed of, in sequence from the object side, a positive third lens whose convex surface faces the image side, and a combined lens formed of a plano-convex lens and a negative meniscus lens.

Lens Data
Surface Number	Radius of Curvature	Inter-Surface Distance	Refractive Index	Abbe Number
Object Plane	∞	10.3427	1.000
1	∞	0.3629	1.768	71.79
2	0.8535	0.3386	1.000
3	∞	0.5444	1.518	75.00
4	∞	0.1089	1.000
5	3.8738	0.6858	1.750	35.33
Stop	∞	0.0544	1.000
7	∞	0.9389	1.700	65.00
8	-1.1501	0.0907	1.000
9	∞	1.0901	1.729	54.68
10	-1.2076	0.5444	1.923	18.90
11	-3.1972	0.6169	1.000
12	∞	0.8165	1.516	64.14
13	∞	0.0363	1.510	64.05
14	∞	0.7258	1.611	50.49
15	∞	0.0000	1.000
Image Plane	∞	0.0000

Example 2

FIG. 4 shows the configuration of an endoscope objective lens according to Example 2, and the lens data thereof are shown below. FIG. 5 shows aberration diagrams of the endoscope objective lens according to this example.
The endoscope objective lens according to this example has the same configuration as the endoscope objective lens according to Example 1.

Lens Data
Surface Number	Radius of Curvature	Inter-Surface Distance	Refractive Index	Abbe Number
Object Plane	∞	10.3437	1.0000
1	∞	0.3629	1.7710	71.79
2	0.8688	0.3462	1.0000
3	∞	0.5444	1.5200	75.00
4	∞	0.1089	1.0000
5	4.0328	0.6824	1.7550	35.33
Stop	∞	0.0544	1.0000
7	∞	0.9529	1.6830	62.00
8	-1.1187	0.0907	1.0000
9	∞	1.0893	1.7320	54.68
10	-1.2089	0.5444	1.9340	18.90
11	-3.1975	0.6170	1.0000
12	∞	0.8166	1.5180	64.14
13	∞	0.0363	1.5120	64.05
14	∞	0.7259	1.6140	50.49
15	∞	0.0000	1.0000
Image Plane	∞	0.0000

Example 3

FIG. 6 shows the configuration of an endoscope objective lens according to Example 3, and the lens data thereof are shown below. FIG. 7 shows aberration diagrams of the endoscope objective lens according to this example.
The endoscope objective lens according to this example has the same configuration as the endoscope objective lens according to Example 1.

Lens Data
Surface Number	Radius of Curvature	Inter-Surface Distance	Refractive Index	Abbe Number
Object Plane	∞	10.3362	1.0000
1	∞	0.3627	1.7710	71.79
2	0.8436	0.3445	1.0000
3	∞	0.5440	1.5200	75.00
4	∞	0.1088	1.0000
5	4.2176	0.6781	1.7550	35.33
Stop	∞	0.0544	1.0000
7	∞	0.9213	1.7440	48.00
8	-1.1926	0.0907	1.0000
9	∞	1.1388	1.7320	54.68
10	-1.1508	0.5440	1.9340	18.90
11	-3.1952	0.6165	1.0000
12	∞	0.8160	1.5180	64.14
13	∞	0.0363	1.5120	64.05
14	∞	0.7253	1.6140	50.49
15	∞	0.0000	1.0000
Image Plane	∞	0.0000

Example 4

FIG. 8 shows the configuration of an endoscope objective lens according to Example 4, and the lens data thereof are shown below. FIG. 9 shows aberration diagrams of the endoscope objective lens according to this example.
The endoscope objective lens according to this example has the same configuration as the endoscope objective lens according to Example 1.

Lens Data
Surface Number	Radius of Curvature	Inter-Surface Distance	Refractive Index	Abbe Number
Object Plane	∞	10.2043	1.0000
1	∞	0.3580	1.7680	71.79
2	0.8504	0.4476	1.0000
3	∞	0.5371	1.5180	75.00
4	∞	0.1074	1.0000
5	3.0183	0.6982	1.7500	35.33
Stop	∞	0.0537	1.0000
7	∞	0.8772	1.7290	54.68
8	-1.3051	0.0895	1.0000
9	∞	1.0204	1.7290	54.68
10	-1.2585	0.5371	1.9230	18.90
11	-3.1544	0.6087	1.0000
12	∞	0.8056	1.5160	64.14
13	∞	0.0358	1.5100	64.05
14	∞	0.7161	1.6110	50.49
15	∞	0.0000	1.0000
Image Plane	∞	0.0000

Example 5

FIG. 10 shows the configuration of an endoscope objective lens according to Example 5, and the lens data thereof are shown below. FIG. 11 shows aberration diagrams of the endoscope objective lens according to this example.
The endoscope objective lens according to this example has the same configuration as the endoscope objective lens according to Example 1.

Lens Data
Surface Number	Radius of Curvature	Inter-Surface Distance	Refractive Index	Abbe Number
Object Plane	∞	10.3379	1.0000
1	∞	0.3627	1.7680	71.79
2	0.7993	0.3627	1.0000
3	∞	0.5441	1.5180	75.00
4	∞	0.1088	1.0000
5	3.5505	0.6936	1.9230	18.90
Stop	∞	0.0907	1.0000
7	∞	0.9136	1.7000	65.00
8	-1.2145	0.0907	1.0000
9	∞	1.0772	1.7290	54.68
10	-1.2237	0.5441	1.9230	18.90
11	-3.1957	0.6166	1.0000
12	∞	0.8162	1.5160	64.14
13	∞	0.0363	1.5100	64.05
14	∞	0.7255	1.6110	50.49
15	∞	0.0000	1.0000
Image Plane	∞	0.0000

Example 6

FIG. 12 shows the configuration of an endoscope objective lens according to Example 6, and the lens data thereof are shown below. Furthermore, FIG. 13 shows aberration diagrams of the endoscope objective lens according to this example.
The endoscope objective lens according to this example has the same configuration as the endoscope objective lens according to Example 1.

Lens Data
Surface Number	Radius of Curvature	Inter-Surface Distance	Refractive Index	Abbe Number
Object Plane	∞	9.8584	1.0000
1	∞	0.3585	1.7680	71.79
2	0.8514	0.4481	1.0000
3	∞	0.5377	1.5180	74.70
4	∞	0.1075	1.0000
5	3.022	0.6990	1.7500	35.33
Stop	∞	0.0538	1.0000
7	∞	0.8783	1.7290	54.68
8	-1.3067	0.0896	1.0000
9	∞	1.0217	1.7290	54.68
10	-1.2601	0.5377	1.9230	18.90
11	-3.1583	0.6005	1.0000
12	∞	0.8066	1.5160	64.14
13	∞	0.0358	1.5100	64.10
14	∞	0.7170	1.5060	50.20
15	∞	0.0000	1.0000
Image Plane	∞	0.0000

Example 7

FIG. 14 shows the configuration of an endoscope objective lens according to Example 7, and the lens data thereof are shown below. Furthermore, FIG. 15 shows aberration diagrams of the endoscope objective lens according to this example.
The endoscope objective lens according to this example has the same configuration as the endoscope objective lens according to Example 1.

Lens Data
Surface Number	Radius of Curvature	Inter-Surface Distance	Refractive Index	Abbe Number
Object Plane	∞	9.8401	1.0000	0.00
1	∞	0.3453	1.7680	71.79
2	0.8264	0.3453	1.0000
3	∞	0.5179	1.5180	75.00
4	∞	0.1036	1.0000
5	3.4154	0.5885	1.7500	35.33
Stop	∞	0.0518	1.0000
7	∞	0.8294	1.7000	65.00
8	-1.1205	0.0863	1.0000
9	∞	1.1011	1.7290	54.68
10	-1.1213	0.5179	1.9230	18.90
11	-3.0418	0.5524	1.0000
12	∞	0.7769	1.5160	64.14
13	∞	0.0345	1.5100	64.05
14	∞	0.6905	1.6110	50.49
15	∞	0.0000	1.0000
Image Plane	∞	0.0000

Example 8

FIG. 16 shows the configuration of an endoscope objective lens according to Example 8, and the lens data thereof are shown below. Furthermore, FIG. 17 shows aberration diagrams of the endoscope objective lens according to this example.
The endoscope objective lens according to this example has a configuration different from that according to Example 1 in that it has a positive plano-convex lens, which serves as a sixth lens and whose convex surface faces the object side, behind the combined lens in the back group.

Lens Data
Surface Number	Radius of Curvature	Inter-Surface Distance	Refractive Index	Abbe Number
Object Plane	∞	9.1946	1.0000
1	∞	0.3226	1.7710	71.70
2	0.7662	0.3549	1.0000
3	∞	0.4839	1.5200	74.44
4	∞	0.0807	1.0000
5	1.8244	0.6291	1.8830	40.76
Stop	∞	0.0484	1.0000
7	∞	0.6130	1.7290	54.68
8	-1.4631	0.0887	1.0000
9	∞	0.7259	1.7290	54.68
10	-1.0872	0.4033	1.9230	18.90
11	-3.4262	0.6593	1.0000
12	3.4956	0.7098	1.5160	64.14
13	∞	0.0323	1.5120	63.80
14	∞	0.6452	1.6140	50.20
15	∞	0.0000	1.0000
Image Plane	∞	0.0000

Example 9

FIG. 18 shows the configuration of an endoscope objective lens according to Example 9, and the lens data thereof are shown below. Furthermore, FIG. 19 shows aberration diagrams of the endoscope objective lens according to this example.
The endoscope objective lens according to this example has the same configuration as Example 8.

Lens Data
Surface Number	Radius of Curvature	Inter-Surface Distance	Refractive Index	Abbe Number
Object Plane	∞	10.0351	1.0000
1	∞	0.3169	1.7680	71.79
2	1.0035	0.2887	1.0000
3	∞	0.6162	1.5180	75.00
4	∞	0.0880	1.0000
5	3.5911	0.5282	1.7500	35.33
Stop	∞	0.0528	1.0000
7	∞	0.8726	1.6700	47.23
8	-1.1315	0.0880	1.0000
9	∞	0.6162	1.7290	54.68
10	-1.1144	0.5282	1.9230	18.90
11	-3.1021	0.7007	1.0000
12	4.9101	0.7922	1.5160	64.14
13	∞	0.0352	1.5100	64.05
14	∞	0.7042	1.6110	50.49
15	∞	0.0000	1.0000
Image Plane	∞	0.0000

Example 10

FIG. 20 shows the configuration of an endoscope objective lens according to Example 10, and the lens data thereof are shown below. Furthermore, FIG. 21 shows aberration diagrams of the endoscope objective lens according to this example.
The endoscope objective lens according to this example has the same configuration as Example 8.

Lens Data
Surface Number	Radius of Curvature	Inter-Surface Distance	Refractive Index	Abbe Number
Object Plane	∞	8.1077	1.0000
1	∞	0.3025	1.8830	40.76
2	0.7741	0.3581	1.0000
3	∞	0.4267	1.5200	74.44
4	∞	0.0711	1.0000
5	1.7943	0.5547	1.8830	40.76
Stop	∞	0.0427	1.0000
7	∞	0.7390	1.5890	61.14
8	-1.1321	0.0782	1.0000
9	∞	0.6869	1.7290	54.68
10	-0.9587	0.4004	1.9230	18.90
11	-3.0212	0.9550	1.0000
12	5.1223	0.6259	1.5160	64.14
13	∞	0.0284	1.5120	63.80
14	∞	0.5690	1.6140	50.20
15	∞	0.0000	1.0000
Image Plane	∞	0.0000

Example 11

FIG. 22 shows the configuration of an endoscope objective lens according to Example 11, and the lens data thereof are shown below. Furthermore, FIG. 23 shows aberration diagrams of the endoscope objective lens according to this example.
The endoscope objective lens according to this example has a configuration different from that according to Example 1 in that a filter is disposed behind the second lens and in that a biconvex lens is used as the fourth lens.

Lens Data
Surface Number	Radius of Curvature	Inter-Surface Distance	Refractive Index	Abbe Number
Object Plane	∞	8.9419	1.0000
1	∞	0.3922	2.1820	33.01
2	1.1808	0.1771	1.0000
3	2.1954	0.6258	1.8880	40.76
4	∞	0.0471	1.0000
5	∞	0.4960	1.5200	75.00
Stop	∞	0.0471	1.0000
7	∞	1.0040	1.7470	49.34
8	-1.3674	0.0784	1.0000
9	6.2130	0.7844	1.8880	40.76
10	-0.7703	0.6040	1.9340	18.90
11	-3.7046	0.5370	1.0000
12	∞	0.7059	1.5180	64.14
13	∞	0.0314	1.5120	64.05
14	∞	0.6275	1.6140	50.49
15	∞	0.0000	1.0000
Image Plane	∞	0.0000

Example 12

FIG. 24 shows the configuration of an endoscope objective lens according to Example 12, and the lens data thereof are shown below. Furthermore, FIG. 25 shows aberration diagrams of the endoscope objective lens according to this example.
The endoscope objective lens according to this example has a configuration different from that according to Example 1 in that a positive meniscus lens is used as the third lens.

Lens Data
Surface Number	Radius of Curvature	Inter-Surface Distance	Refractive Index	Abbe Number
Object Plane	∞	6.6534	1.0000
1	∞	0.2918	1.7680	71.79
2	1.1883	0.2335	1.0000
3	∞	0.3502	1.5180	75.00
4	∞	0.0700	1.0000
5	2.2526	0.4727	1.7500	35.33
Stop	∞	0.0350	1.0000
7	-4.4676	0.6494	1.9300	45.00
8	-1.1325	0.0584	1.0000
9	∞	0.8238	1.7290	54.68
10	-0.8710	0.4669	1.9230	18.90
11	-2.0567	0.4253	1.0000
12	∞	0.6420	1.5160	64.14
13	∞	0.0233	1.5100	64.05
14	∞	0.4669	1.6110	50.49
15	∞	0.0000	1.0000
Image Plane	∞	0.0000

Example 13

FIG. 26 shows the configuration of an endoscope objective lens according to Example 13, and the lens data thereof are shown below. The endoscope objective lens according to this example has the same configuration as the endoscope objective lens according to Example 1 from the first lens to the combined lens, and an optical-path changing device P, which changes light in a perpendicular direction, is disposed on the object side of the image acquisition device.

Lens Data
Surface Number	Radius of Curvature	Inter-Surface Distance	Refractive Index	Abbe Number
Object Plane	∞	10.3427	1.0000
1	∞	0.3629	1.7680	71.79
2	0.8535	0.3386	1.0000
3	∞	0.5444	1.5180	75.00
4	∞	0.1089	1.0000
5	3.8738	0.6858	1.7500	35.33
Stop	∞	0.0544	1.0000
7	∞	0.9389	1.7000	65.00
8	-1.1501	0.0907	1.0000
9	∞	1.0901	1.7290	54.68
10	-1.2076	0.5444	1.9230	18.90
11	-3.1972	0.4595	1.0000
12	∞	1.1000	1.8830	40.76
13	∞	1.1000	1.8830	40.76
(Reflection Plane)
14	∞	0.0000	1.0000
Image Plane	∞	0.0000

Table 1 shows the respective parameters in Examples 1 to 13 and the values in Conditional Expression (1) to (3).

{Table 1}

Example	Focal Length of Entire System	Conditional Expression (1)			Conditional Expression (2)			Conditional Expression (3)
Example	Focal Length of Entire System	n3	ν3	-ν3/12 +5.5	f3	r3	\|f3/r3\|	df	Ih	df/Ih
1	1.000	1.7	65	0.0833	1.643	-1.1501	1.4286	1.6777	0.9399	1.7849
2	1.000	1.68	62	0.3333	1.6389	-1.1187	1.465	1.6819	0.94	1.7893
3	1.000	1.74	48	1.5	1.6036	-1.1926	1.3447	1.6755	0.9393	1.7837
4	1.000	1.73	54.68	0.9433	1.7898	-1.3051	1.3714	1.7902	0.9273	1.9305
5	1.000	1.7	65	0.0833	1.735	-1.2145	1.4286	1.7092	0.9395	1.8193
6	1.000	1.73	54.68	0.9433	1.7843	-1.3067	1.3655	1.7924	0.9285	1.9305
7	1.000	1.7	65	0.0833	1.6007	-1.1205	1.4286	1.5553	1.0341	1.504
8	1.000	1.73	54.68	0.9433	2.0065	-1.4631	1.3714	1.5486	0.8275	1.8713
9	1.000	1.67	47.23	1.5642	1.6888	-1.1315	1.4925	1.5211	0.9771	1.5568
10	1.000	1.59	61.14	0.405	1.9217	-1.1321	1.6974	1.4107	0.7297	1.9333
11	1.000	1.74	49.34	1.3883	1.8311	-1.3674	1.3391	1.346	0.7675	1.7536
12	1.000	1.93	45	1.75	1.4832	-1.1325	1.3096	1.1264	0.6046	1.8629
13	1.000	1.7	65	0.0833	1.643	-1.1501	1.4286	1.6777	0.9399	1.7849

(Additional Items)

An invention having the following configurations is derived from the above-described examples.

(Additional Item 1)

An endoscope objective lens comprising, in sequence from an object side: a front group; an aperture stop; and a back group, wherein the front group includes, in sequence from the object side, a negative first lens whose concave surface faces an image side, a positive second lens whose convex surface faces the object side and whose flat surface or concave surface is located on the image side, and a filter, and the back group includes, in sequence from the object side, a positive third lens whose convex surface faces the image side, and a combined lens formed of a plano-convex lens or a biconvex lens and a negative meniscus lens and satisfies the following Conditional Expression (1) $n 3 > - ν 3 / 12 + 5.5$
where n3 is the refractive index of the third lens, and ν3 is the Abbe number of the third lens.

(Additional Item 2)

An endoscope objective lens comprising, in sequence from an object side: a front group; an aperture stop; and a back group, wherein the front group includes, in sequence from the object side, a negative first lens whose concave surface faces an image side, a positive second lens whose convex surface faces the object side and whose flat surface or concave surface is located on the image side, and a filter disposed between the first lens and the second lens, and the back group includes, in sequence from the object side, a positive third lens whose convex surface faces the image side, and a combined lens formed of a plano-convex lens or a biconvex lens and a negative meniscus lens.

(Additional Item 3)

The endoscope objective lens according to Additional Item 2, wherein the following Conditional Expression (1) is satisfied $n 3 > - ν 3 / 12 + 5.5$
where n3 is the refractive index of the third lens, and ν3 is the Abbe number of the third lens.

(Additional Item 4)

The endoscope objective lens according to any one of Additional Items 1 to 3, wherein the following Conditional Expressions (2) and (3) are satisfied $|f 3 / r 3| > 1.3$
$2.0 > df / Ih > 1.5$
where f3 is the focal length of the third lens, r3 is the image-plane-side radius of curvature of the third lens, df is the sum of the thickness of an optical element and the inter-surface distance from the apex of the concave surface of the first lens to the aperture stop, and Ih is the maximum image height.

(Additional Item 5)

The endoscope objective lens according to any one of Additional Items 1 to 4, wherein the back group includes a plurality of positive lenses.

(Additional Item 6)

The endoscope objective lens according to any one of Additional Items 1 to 5, wherein the filter is an optical filter, which is not limited to an infrared-cut filter or a color-correcting filter.

(Additional Item 7)

The endoscope objective lens according to any one of Additional Items 1 to 6, wherein the first lens is made of a material having chemical resistance or sterilization resistance.

(Additional Item 8)

The endoscope objective lens according to any one of Additional Items 1 to 7, comprising an optical-path changing device, such as a prism or the like.

(Additional Item 9)

An endoscope comprising the endoscope objective lens according to any one of Additional Items 1 to 8.

{Reference Signs List}

1: endoscope objective lens
FG: front group
BG: back group
FL: filter
L1: first lens
L2: second lens
L3: third lens
L4: fourth lens (plano-convex lens or biconvex lens)
L5: fifth lens (negative meniscus lens)
E45: combined lens
S: aperture stop
P: optical-path changing device

Claims

An endoscope objective lens comprising, in sequence from an object side:
a front group;

an aperture stop; and

a back group, wherein

the front group includes, in sequence from the object side, a negative first lens whose concave surface faces an image side, a positive second lens whose convex surface faces the object side and whose flat surface or concave surface is located on the image side, and a filter, and

the back group includes, in sequence from the object side, a positive third lens whose convex surface faces the image side, and a combined lens formed of a plano-convex lens or a biconvex lens and a negative meniscus lens and satisfies the following Conditional Expression (1) $n 3 > - ν 3 / 12 + 5.5$

where n3 is the refractive index of the third lens, and ν3 is the Abbe number of the third lens.
An endoscope objective lens comprising, in sequence from an object side:
a front group;

an aperture stop; and

a back group, wherein

the front group includes, in sequence from the object side, a negative first lens whose concave surface faces an image side, a positive second lens whose convex surface faces the object side and whose flat surface or concave surface is located on the image side, and a filter disposed between the first lens and the second lens, and

the back group includes, in sequence from the object side, a positive third lens whose convex surface faces the image side, and a combined lens formed of a plano-convex lens or a biconvex lens and a negative meniscus lens.
The endoscope objective lens according to Claim 2, wherein the following Conditional Expression (1) is satisfied $n 3 > - ν 3 / 12 + 5.5$
where n3 is the refractive index of the third lens, and ν3 is the Abbe number of the third lens.
The endoscope objective lens according to any one of Claims 1 to 3, wherein the following Conditional Expressions (2) and (3) are satisfied $|f 3 / r 3| > 1.3$
$2.0 > df / Ih > 1.5$
where f3 is the focal length of the third lens, r3 is the image-plane-side radius of curvature of the third lens, df is the sum of the thickness of an optical element and the inter-surface distance from the apex of the concave surface of the first lens to the aperture stop, and Ih is the maximum image height.
An endoscope comprising the endoscope objective lens according to any one of Claims 1 to 4.